Optical systems and refractive index-matching compositions

ABSTRACT

Optical systems such as, for example, optical switches, are disclosed comprising a solid component and a refractive index-matching liquid composition interfaced with the solid component. The liquid composition has a refractive-index that is substantially equal to that of the solid component. In one approach the liquid composition is a saturated cyclic compound consisting essentially of carbon and hydrogen and optionally oxygen such as, e.g., cyclic alkanes, alcohols or ketones. In another approach the liquid composition is benzene substituted with one or more electron-donating groups attached directly to the ring and one of more fluoro groups attached to the ring or to the electron-donating groups. In yet another approach the liquid composition is a combination comprising one or more of benzene or substituted benzene and optionally at least one of an alkane or substituted alkane having a boiling point less than about 130° C. Also disclosed are methods for preparing a liquid composition having a predetermined refractive index at a predetermined temperature and methods for transmitting optical signals.

This application is a continuation of U.S. patent application Ser. No.10/010,810 filed Nov. 13, 2001, now U.S. Pat. No. 6,890,619, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to the area of devices having a liquidinterfaced with a solid component. In particular, the invention isconcerned with optical systems and liquids for refractiveindex-matching. In one aspect the invention concerns methods forpreparing a liquid composition for use in refractive index-matching.

Traditionally, signal exchanges within telecommunications networks anddata communications networks have been accomplished by transmittingelectrical signals via electrically conductive lines. However, analternative medium of data exchange is the transmission of opticalsignals through optical fibers. Since a light beam can be easilygenerated and modulated so as to carry much more information than ispossible with an electrical signal, light beams have a distinctadvantage in communicating information between two or more locations.High quality, low loss optical fibers are now readily available so thatoptical fibers are now being used widely to define optical paths forsuch light beams. Fiber optical switches may be used to control routingof optical beams along alternative fiber optical paths. Equipment forefficiently generating and transmitting the optical signals has beendesigned and implemented, and optical switches have been designed foruse in telecommunications and data communications networks.

There are many situations in optical systems where a liquid whoserefractive index is approximately equal, or equal, to that of a solidcomponent of the optical system, is useful such as, for example, opticalswitches. Most refractive index data is available for the wavelength586.26 nanometers (nm), also known as the sodium D line. For a smallernumber of materials, information is also available at a limited numberof other visible wavelengths such as 706.52 nm, 667.81 nm, 656.28 nm,546.07 nm, 501.57 nm, 486.13 nm and 435.83 nm. However, there is verylittle data on the refractive index of liquids in the wavelength rangesrelevant to the communications industry, which are typically 1270 to1350 nm and 1470 to 1620 nm. Techniques of estimation must be used formost compounds, and several commonly used approximations (e.g.,Sellmeier equation) are not accurate due to the combined effects ofultraviolet (UV) and infra-red (IR) absorption bands. It is, therefore,not a simple matter of looking up the refractive index of a potentialliquid. Experimental determination is also difficult requiring specialapparatus.

Preferably, the liquid for refractive index-matching does not have atomsof chlorine, bromine or iodine as certain compounds with these elementsare known to deplete the ozone layer of the earth. The liquid preferablydoes not contain toxic elements such as, for example, lead, tin,mercury, or other heavy metals. However, it is difficult to findcompounds that have a low boiling point and sufficiently high refractiveindex for many applications without resorting to materials that containchlorine, bromine, iodine, heavy metals, sulfur, selenium, tellurium orarsenic. Without these special refractive index enhancers, therefractive index largely depends on the number of atoms per unit volume.This depends on the number of atoms per molecule and the intermolecularforces that draw the molecules towards each other. However, usingmaterials with increasing numbers of atoms per molecule and/or strongerintermolecular forces invariably increases the boiling point or lowersthe vapor pressure.

There remains a need for refractive index matching liquids that have alow boiling point and sufficiently high refractive index forapplications in optical systems and other areas where the refractiveindex of a liquid must match or approximate that of a solid componentwith which it is interfaced. The liquid should not comprise a materialthat contains chlorine, bromine, iodine, heavy metals, sulfur (in mostcases), selenium, tellurium or arsenic.

SUMMARY OF THE INVENTION

The present invention provides liquids for use in devices where theliquid is interfaced with a solid component of the device. The presentliquids have a refractive index that is equal to, or substantially equalto, the refractive index of the solid component. The liquid may be movedwith respect to the solid component or vice versa without significantdamage to the solid component. The present liquids are stable at thetemperatures of use and do not change their optical or physicalparameters to any significant degree. The liquids of the invention donot, to any substantial degree, decompose, polymerize, form soliddeposits or react with any of the materials with which it will be incontact such as, e.g., components of an optical device, including metalsin the presence of light and heat. Furthermore, the liquids have lowabsorption of light and relatively low toxicity. The present refractiveindex-matching liquids match well over a range of wavelengths such as,for example, the wavelength ranges of about 1250 to about 1320 nm andabout 1470 to about 1600 nm. These wavelength ranges are typically usedfor optical communications and, accordingly, the present liquids areuseful for index-matching in optical communications devices.

For ease of handling and manufacturing, the present liquids do not reactappreciably with air nor absorb any component of air such as, e.g.,water vapor and carbon dioxide, rapidly enough to cause problems. Theinterface between a liquid of the invention and a solid component of adevice has negligible reflection and refraction, which is beneficial foroptical systems. In addition, diffraction effects can be minimized inmany cases with the present liquids.

The present invention provides a device comprising a solid component anda liquid composition interfaced therewith. The liquid composition has arefractive-index that is substantially equal to that of the solidcomponent and is selected from the group consisting of (i) saturatedcyclic compounds consisting essentially of carbon and hydrogen andoptionally oxygen, (ii) benzene substituted with one or moreelectron-donating groups attached directly to the ring and one of morefluoro groups attached to the ring or to the electron-donating groups,and (iii) a combination comprising one or more of benzene or substitutedbenzene and optionally at least one of an alkane or substituted alkanehaving a boiling point less than about 130° C.

The present invention provides an optical system comprising a solidcomponent of an optical system and the aforementioned liquid compositioninterfaced therewith where the liquid composition has a refractive-indexthat is substantially equal to that of the solid component.

The present invention provides a method of preparing a liquidcomposition having a predetermined refractive index at a predeterminedtemperature. The method comprises combining a first reagent having arefractive index that is higher than the predetermined refractive indexat the predetermined temperature and a boiling point that is less thanabout 100° C. with a second reagent having a refractive index that islower than the refractive index of the first reagent and a boiling pointof less than about 130° C. The first reagent and the second reagent arecombined in amounts effective to obtain the liquid composition having apredetermined refractive index. Optionally, the liquid composition maycomprise additional reagents, each having a respective refractive indexand boiling point, in an amount sufficient to obtain the liquidcomposition having a predetermined refractive index at the predeterminedtemperature.

The present invention provides an optical switch comprising opticalwaveguides that are formed in a substrate and intersect each other, acavity having a wall surface at a predetermined angle from the opticalaxis of the optical waveguide positioned at the intersection of theoptical waveguides, and a refractive index-matching liquid in thecavity. The refractive index-matching liquid is one of theaforementioned liquid compositions.

The present invention provides a method for matching the refractiveindex of a solid component of a device. A solid component of the deviceis contacted with a liquid composition having a refractive index that issubstantially equal to that of the solid component. The liquidcomposition is selected from the aforementioned groups.

The present invention provides a method for transmitting optical signal.In the method an optical signal is generated and transmitted along apathway of an optical device. A refractive index-matching liquidselected from the aforementioned groups is moved into and out ofintersection with the pathway to control the transmission of the opticalsignal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a top view of an optical switching element.

FIG. 2 is a matrix of switching elements of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention provides liquids for use indevices where the liquid is interfaced with a solid component of thedevice. The present liquids have a refractive index that is equal to, orsubstantially equal to, the refractive index of the solid component. Thepresent refractive index-matching liquids match well over a range ofwavelengths such as, for example, the wavelength ranges of about 1250 toabout 1320 nm and about 1470 to about 1600 nm. These wavelength rangesare typically used for optical communications and, accordingly, thepresent liquids are useful for index-matching in optical communicationsdevices.

In one aspect the liquids of the present invention have application tosituations where a liquid for refractive index matching must be removedby vaporization, i.e., evaporation or boiling. An example is a waveguideoptical cross-connect switch as described by Fouquet, et al., infra. Theliquids of the invention have relatively low boiling points and energyrequirements to vaporize. The liquids of the invention also exhibit arelatively rapid change of vapor pressure with temperature. The lattercharacteristic is advantageous in situations when a vapor bubble must bemaintained against pressure. The refractive index matching liquids ofthe invention have a vapor composition that is not substantiallydifferent from the liquid composition. Accordingly, changes in therefractive index due to separation processes similar to distillation areavoided in the present invention.

The refractive index matching liquids of the invention may also be usedin applications where the matching liquid is exposed to the atmospherefor extended periods. In those situations, a high boiling point or,equivalently, a low vapor pressure, is preferred. This is particularlyimportant for mixtures of liquids where preferential evaporation of onecomponent of a mixture would lead to a change in composition and,therefore, in refractive index.

Examples of devices that comprise a solid component that is interfacedwith a refractive index matching liquid of the invention are nextdiscussed by way of illustration and not limitation. As one skilled inthe art will appreciate, the invention has application to any devicewherein a liquid is interfaced with a solid component and it is desiredthat the refractive index of the liquid match the refractive index ofthe solid component.

One type of device to which the present invention may be applied is adevice employed in optical systems. For example, optical switches areemployed for setting and switching optical paths in opticalcommunication systems and the like. Often, optical paths are switched bychanging the conditions of reflection and transmission of light as aresult of moving a liquid in a cavity such as, for example, a groove orgap, formed at some point of the optical path. The optical switchesoften comprise optical waveguides that are formed in a substrate andintersect each other such as by crossing each other, forming a Tintersection and so forth. The optical switches also comprise a cavityhaving a wall surface at a predetermined angle crossed from the opticalaxis of optical waveguide and positioned at the intersection of theoptical waveguides, and a refractive index-matching liquid in thecavity.

The liquid in the cavity may be sealed in the cavity or it may beexposed to the outside. One method proposed for switching optical pathsinvolves pouring a refractive index-matching liquid into a cavity formedin a crossing point of optical waveguides arranged in a lattice pattern.In another method proposed for switching optical paths, bubbles aregenerated by an electrolytic action to remove a liquid in a cavity. Whenthe bubbles disappear by recombination using catalytic electrodes,liquid refills the cavity. In both of the above methods, the liquid isexposed to the outside. As such, the liquid is subject to evaporationand the like.

As mentioned above, sealed or sealable devices are known. One approachis discussed in U.S. Pat. No. 5,699,462 (Fouquet, et al.), which isdiscussed in detail below. A switching element defines a transmittingstate and a reflecting state for a pair of intersecting waveguides thathave a gap at their intersection. The waveguides are part of a sealedsystem wherein in the transmitting state index-matching liquid fills thegap, enabling light to continue in the original waveguide direction.

In another type of optical switch, movement of liquid to be used forswitching optical paths is performed by heating the liquid through aheater arranged near a cavity. In one approach an optical switchcomprises an optical waveguide formed on a substrate and a gap in theform of a groove that is formed so as to cross a core of the waveguide.The gap stores a refractive index-matching liquid have the samerefractive index as that of the core. A means for regulating thetemperature of liquid such as, for example, a thermoelectric coolingelement, is arranged in proximity to the gap. This element forcefullyheats and vaporizes the liquid in the gap or forcefully cools andcondenses the liquid.

As mentioned above, the present invention provides liquid compositionsthat have refractive indices that are equal to, or substantially equalto, the refractive indices of solid components of optical systems. Bythe phrase “substantially equal to” is meant that the refractive indexis at least within about 0.01 units, preferably, about 0.001 units, ofthe refractive index of the solid component. Desirably, the refractiveindex is identical to the refractive index of the solid component.

The liquid compositions comprise primarily three different types. In oneembodiment the liquid composition comprises a saturated cyclic compoundconsisting essentially of carbon and hydrogen and optionally oxygen(Refractive Index Matching Liquid—Type 1). In another embodiment theliquid composition comprises a benzene substituted with one or moreelectron-donating groups attached directly to the ring and one or morefluoro groups attached to the ring or to the electron-donating groups(Refractive Index Matching Liquid—Type 2). In yet another embodiment theliquid composition is a combination comprising one or more of benzene orsubstituted benzene and optionally at least one of an alkane orsubstituted alkane having a boiling point less than about 130° C.(Refractive Index Matching Liquid—Type 3). The aforementionedembodiments will next be discussed in detail.

Refractive Index Matching Liquid—Type 1

As mentioned above, one liquid composition in accordance with thepresent invention comprises a saturated cyclic compound consistingessentially of carbon and hydrogen and optionally oxygen. By the term“saturated” is meant that the compound does not contain anycarbon-carbon double bonds or triple bonds. By the term “cyclic” ismeant that the compound comprises one or more rings. Usually, the numberof rings is 1 to about 4, more usually, 1 to 2. For the most part theatoms of the rings are carbon atoms. However, it is within the purviewof the invention that one or more of the atoms may be oxygen ornitrogen. The number of each of the aforementioned atoms in thesaturated cyclic compound is dependent on the number of rings, thenature of the atoms present, and the desired refractive index of theliquid composition. The number of atoms of carbon in the saturatedcyclic compound is about 4 to about 100, usually, about 5 to about 60,more usually, about 5 to about 12. Lower number of atoms in the compoundusually relates to a lower boiling point. For a compound that has onlyone ring, the number of carbon atoms in the ring is usually about 4 toabout 20, more usually about 5 to about 16. In this embodiment thesaturated cyclic compound may be, for example, a saturated monocyclichydrocarbon such as cyclobutane, cyclopentane, cyclohexane,cycloheptane, cyclooctane, cyclononane, cyclodecane, cyclohexadecane,and the like.

When more than one ring is present, the rings may be fused rings orspiro rings. In this embodiment the saturated cyclic compound may be,for example, a saturated fused ring or spiro ring hydrocarbon such asbicyclohexane, bicycloheptane, bicyclooctane, bicyclononane, adamantane,decahydronaphthalene, and the like and including the isomeric formsthereof such as cis and trans isomers where appropriate.

On the other hand, where more than one ring is present, the rings may beconnected to one another through a bond or a linking group. In a sensethe ring is substituted with a substituent that comprises another ringlinked to the first ring by a bond or a linking group. The nature of thelinking group is in keeping with the saturated nature of theaforementioned liquid compositions. Accordingly, the linking group isusually an alkylene chain. “Alkylene” means a branched or unbranchedsaturated divalent hydrocarbon radical containing 1 to 30 or more carbonatoms, such as methylene, ethylene, propylene, 2-methylpropylene,1,2-dimethylpropylene, pentylene, and the like. It is within the scopeof the present invention that the linking group may comprise 1 to about10 oxygen atoms usually in the form of an alcohol or keto functionality.

The number of hydrogen atoms present is dependent on the number ofcarbon atoms and, optionally, the number of oxygen atoms, in thesaturated cyclic compound. As a general rule, the number of hydrogenatoms is that necessary to satisfy the valencies of carbon andoptionally oxygen atoms present in the saturated cyclic compound.

As may be seen from the above discussion, the saturated cyclic compoundmay be unsubstituted, thus comprising no substituents on the ring orrings. On the other hand, it may be substituted; that is, it maycomprise one or more substituents on the ring or rings of the cycliccompound. Substituted generally means that a hydrogen atom of thesaturated cyclic compound has been replaced by another atom. Inaccordance with the invention the substituents usually consistessentially of carbon and hydrogen and optionally oxygen. Oxygen isnormally present as an alcohol or a ketone functionality. Thesubstituents may be alkyl, alkoxy, hydroxy, keto, or alkyl or alkoxysubstituted with one or more substituents such as hydroxy, keto, alkoxyand the like. By the term “alkyl” is meant a branched or unbranchedsaturated monovalent hydrocarbon radical containing 1 to 30 or morecarbon atoms, such as methyl, ethyl, propyl, butyl, pentyl, hexyl,heptyl, octyl, nonyl, decyl, undecyl, dodecyl, and so forth, andincluding the normal, secondary, tertiary, and the like, forms thereof.The number of alkyl substituents may be 1 to about 10, usually, 1 toabout 3. The number of hydroxy substituents may be 1 to about 5,usually, 1 to about 3. The number of keto substituents is usually about1 to about 3, usually, 1 to 2.

The saturated cyclic compound in accordance with this embodiment of theinvention usually has a refractive index of about 1.40 to about 1.49,usually, about 1.44 to about 1.475, at 586.26 nanometers (nm). Themelting point of the saturated cyclic compound is usually below about30° C., in some instances below about 15° C., and in other instancesbelow about 0° C.

The refractive index of a material is defined as the ratio of the speedof light in a vacuum to the speed of light in the material. However,when light travels in an optical fiber or waveguide, the light travelsthrough both the core and the cladding, which have different refractiveindices. The distribution of light intensity in the guided mediumdepends on the mode of the light. In general, a guided medium has anumber of modes, but a commonly used type of fiber is manufactured so asto allow only one mode (single-mode fiber). The effective refractiveindex of a particular mode of a guided medium is a function of thegeometry of the medium (shape, dimensions) and the refractive indices ofthe core and cladding regions. It should be noted that the “index” of afiber or waveguide is frequently determined by the length of time ashort pulse of light requires to travel through a known distance of thematerial. This is referred to as the group index, which concerns therate of a collection of different wavelengths of light forming a pulse.It is not the phase index, which is relevant for a single wavelength'sphase speed. It is the phase index that must be matched to preventreflection at an interface of two materials.

The relation between the group index, N, and the phase index, n, is:N=n−λdn/dλwherein λ is wavelength of the light. The quantity that must be matchedto prevent reflection at an interface is, therefore, the effective phaseindex of the guided medium (waveguide), either measured experimentallyor calculated from the geometry and material constants of the guidedmedium. For rectangular silica waveguides, 8 μm by 16 μm, this index at1550 nm is approximately 1.4475, where bulk silica has a refractiveindex of 1.4402.

The above liquid compounds are normally comprised of a single chemicalentity and, therefore, have the advantage of not changing theirrespective refractive index due to evaporation or other processes thatmight change the composition of a mixture. In addition, the purity ofthe above compounds may be checked easily by measuring their index ofrefraction at the sodium D line by conventional means such as, forexample, using an Abbe refractometer.

In one embodiment of this aspect of the invention, the liquidcomposition is a cycloalkane that is unsubstituted or substituted withalkyl, cycloalkyl, hydroxy, keto, alkyl or cycloalkyl substituted withhydroxy or keto, fused cycloalkyl, fused cycloalkyl substituted withhydroxy or keto, spiro cycloalkyl, spiro cycloalkyl substituted withhydroxy or keto.

Examples, by way of illustration and not limitation, of suitablesaturated cyclic compounds in accordance with the present invention areset forth in Table 1.

TABLE 1 n_(D) ²⁰ Saturated Hydrocarbons, one ring: Cycloheptane 1.4455t-Butylcyclohexane 1.4470 1,1,2-trimethylcycloheptane 1.4527Butylcyclooctane 1.4585 Cyclooctane 1.4586 Cyclononane 1.4666Cyclodecane 1.4716 Cyclohexadecane 1.4730 Saturated Hydrocarbons, tworings: Cyclopentylcyclohexane 1.4725 1,1-Dicyclohexylbutane 1.4750Dicyclohexylmethane 1.4752 1,3-Dicyclohexyl-2-Methypropane 1.47561,3-Dicyclohexylpropane 1.4756 Bicyclohexyl 1.47661,2-Dicyclohexylpropane 1.4790 1,1-Dicyclohexylpropane 1.48601,1-Dicyclohexylethane 1.4887 2,2-Dicyclohexylpropane 1.4910 SaturatedHydrocarbons, fused or spiro rings: Bicyclo[3.2.0]Heptane, cis 1.4532Bicyclo[4.1.0]Heptane, cis 1.4564 Spiro[3.5]Nonane 1.4581Bicyclo[4.2.0]Octane, cis 1.4613 Bicyclo[3.3.0]Octane, trans 1.4625Bicyclo[3.3.0]Octane, cis 1.4629 Bicyclo[4.3.0]nonane, trans 1.4643trans-decahydronaphthalene (Bicyclo[4.4.0]decane) 1.4697Bicyclo[4.3.0]nonane, cis 1.4714 1,3 Dimethyl adamantane 1.4780cis-decahydronaphthalene (Bicyclo[4.4.0]decane) 1.4811 Alcoholderivatives: Cyclopentanol 1.4520 4-methylcycloheptanol 1.4574Cyclopentylethanol 1.4577 Cyclopentylmethanol 1.45801-Methylcyclohexanol 1.4585 2,6-Diethylcyclohexanol 1.4600 Menthol(l)(2-isopropyl-5-methylcyclohexanol) 1.4600 trans-2-Methylcyclohexanol1.4610 cis-4-Methyl cyclohexanol 1.4610 (1 S,2S,5R)-(+)-Neomenthol1.4610 Menthol (dl)(2-isopropyl-5-methylcyclohexanol) 1.46153,4-Dimethylcyclohexanol 1.4620 4-Ethylcyclohexanol 1.46254-propylcyclohexanol, cis 1.4632 2-Ethylcyclohexanol 1.4640 Cyclohexanemethanol 1.4644 cis-2-Ethylcyclohexanol 1.4646 2-Cyclohexylethanol1.4647 (±)-cis-2-Methylcyclohexanol 1.4650 (±)-1-Cyclohexane ethanol1.4650 2,3-Dimethylcyclohexanol 1.4650 Cyclohexane methanol 1.4650Cyclohexanol 1.4650 2,3-Dimethylcyclohexanol 1.4653(±)-cis-2-Methylcyclohexanol 1.4655 α-methylcyclohexylmethanol 1.46563-Cyclohexyl-1-propanol 1.4660 4-Cyclohexyl-1-butanol 1.4660cis-2-Isopropylcyclohexanol 1.4665 Dihydroterpineol 1.4670cis-4-Isopropylcyclohexanol 1.4671 1-methylcycloheptanol, 1.46771,2,2-trimethylcyclohexanol, 1.4680 1,2,2-Trimethylcyclohexanol 1.46801-(1-Methylethyl)cyclohexanol 1.4683 cis-2-methylcyclohexylmethanol1.4689 2-methylcycloheptanol 1.4710 Cycloheptylmethanol 1.4740cis-3-Methylcyclohexanol 1.4752 Cycloheptanol 1.4760 Cyclooctanol 1.4860[1,1′-Bicyclopentyl]-2-ol 1.4880 Cyclododecylmethanol 1.49101,3-Cyclohexyldimethanol 1.4912 1,3-Dimorphoyl-2-propanol 1.4980 Ketonederivatives: 2-Methylcyclohexanone 1.4480 Menthone(l) 1.44984-Ethylcyclohexanone 1.4520 2-t-Butylcyclohexanone 1.4570 Cycloheptanone1.4608 4-(1,1-dimethylpropyl)cyclohexanone 1.4677 Cyclooctanone 1.4694Cyclononanone 1.4725 Octahydro-2H-inden-2-one 1.4755[1,1′-bicyclopentyl]-2-one 1.4763 6,6 methyl-(1R)di-bicyclo[3.1.1]heptan-2-one 1.4787 Cyclodecanone 1.4806[1,1′-bicyclohexyl]-2-one 1.4902Refractive Index Matching Liquid—Type 2

As mentioned above, another embodiment of a liquid composition of theinvention that has refractive indices that are equal to, orsubstantially equal to, the refractive indices of solid components ofoptical systems comprises a substituted benzene, usually a fluorinatedbenzene, which optionally may comprise an electron-donating group. Thesecompounds usually have a refractive index of about 1.46 to about 1.49 at586.26 nanometers (nm). The melting point of these compounds is usuallybelow about 5° C., in some instances below about 20° C., and in otherinstances below about 30° C. The number of fluoro (derived fromfluorine) groups on the benzene or on an electron-donating group and thenumber of electron-donating groups in general are selected to achieve arefractive index for the resulting substituted benzene in theaforementioned range. The above liquid compounds are normally comprisedof a single chemical entity and, therefore, have the advantage of notchanging their respective refractive index due to evaporation or otherprocesses that might change the composition of a mixture.

The substituted benzene compounds of this aspect of the inventioninclude one or more, usually, 1 to about 5, more usually, 1 to about 3,fluoro groups attached to the ring of the benzene or toelectron-donating groups, which may be substituted on the benzene ring.In general, the benzene is substituted with one or more, usually, 1 toabout 2, electron-donating groups attached directly to the benzene ring.The electron-donating group is usually a substituent which, when boundto the benzene, is capable of polarizing the molecule such that theelectron-donating group becomes electron poor and positively chargedrelative to another portion of the molecule, i.e., has reduced electrondensity. Such groups include, by way of illustration and not limitation,alkyl, alkoxy, hydroxy, amino (with the proviso that the compound beliquid) and the like. By the term “alkoxy” is meant a moiety thatcomprises an alkyl group-linked to oxygen, which is attached to thebenzene ring. Alkoxy includes, for example, methoxy, ethoxy, propoxy,butoxy, pentoxy, hexoxy, heptoxy, octoxy, nonoxy, decoxy, undecoxy,dodecoxy, and so forth, and including the normal, secondary, tertiary,and the like, forms thereof.

Examples, by way of illustration and not limitation, of suitablesubstituted benzene compounds in accordance with the present inventionare set forth in Table 2.

TABLE 2 Substituted benzene: n_(D) ²⁰ 3-(Trifluoromethyl)benzyl alcohol1.4600 α-(Trifluoromethyl)benzyl alcohol 1.46103-(Trifluoromethyl)phenethyl alcohol 1.4630 3,5-Difluoroanisole 1.4660Fluorobenzene 1.4677 2-(Trifluoromethyl)benzyl alcohol 1.46803,4-Difluoroanisole 1.4689 3-Fluorotoluene 1.4691 4-Fluorotoluene 1.46922-(Trifluoromethyl) phenethyl alcohol 1.4700 2-Fluorotoluene 1.47042,4-Difluoroanisole 1.4705 2′,4′,5′-Trifluoroacetophenone 1.47202-Fluoro-m-xylene 1.4790 2-Fluoro-1,3,5-Trimethylbenzene 1.48094-Fluorophenetole 1.4826 3-Fluoro-o-xylene 1.4860Refractive Index Matching Liquid—Type 3

In another embodiment of a liquid composition for refractive indexmatching, the liquid composition is a combination comprising one or moreof benzene or substituted benzene or thiophene or substituted thiopheneand optionally at least one of an alkane or substituted alkane having aboiling point less than about 130° C. The weight percent of benzene orsubstituted benzene or thiophene or substituted thiophene in thecombination is about 10% to about 95%, usually, about 30% to about 90%.The weight percent is dependent on the refractive indices and boilingpoints of the benzene or substituted benzene or thiophene or substitutedthiophene and the alkane or substituted alkane in the combination. Theliquid composition in accordance with this embodiment of the inventionusually has a refractive index of about 1.49 to about 1.60 at 586.26nanometers (nm).

Accordingly, in one aspect of the invention a liquid composition isprepared having a predetermined refractive index at a predeterminedtemperature. The method comprises combining a first reagent, forexample, a benzene or thiophene, with a second reagent, for example, asubstituted benzene (or benzene where the first reagent is not benzene)or an alkane or substituted alkane. The first reagent has a refractiveindex that is higher than the predetermined refractive index at thepredetermined temperature and a boiling point that is less than about100° C., usually, less than about 85° C. The second reagent has arefractive index that is lower than the refractive index of the firstreagent and a boiling point of less than about 130° C., usually, lessthan about 85° C. The first reagent and the second reagent are combinedin amounts effective to obtain a liquid composition having apredetermined refractive index. Usually, these amounts are obtainedempirically. For example, the first reagent may be benzene orsubstituted benzene and the second reagent may be benzene or substitutedbenzene or an alkane or substituted alkane and the predeterminedrefractive index may be in the range set forth above. In anotherexample, the first reagent may be thiophene or substituted thiophene andthe second reagent may be an alkane or substituted alkane and thepredetermined refractive index may be in the range set forth above. Itshould be noted that, although thiophene does contain a sulfur atom,thiophene is acceptable in the present compositions because its odor isnot offensive and its toxicity is relatively low.

It is within the purview of the present invention to include in theabove composition additional reagents, each having a known refractiveindex and a boiling point, in an amount sufficient to obtain the liquidcomposition having a predetermined refractive index. Such additionalreagents may be the aforementioned benzene or substituted benzene oralkane or substituted alkane. Again, the amounts of the additionalreagents are determined empirically.

As mentioned above, the benzene may be unsubstituted or substituted withone or more alkyl groups, fluoro groups, fluoroalkyl groups, alkoxygroups, and the like. The alkyl groups comprise about 1 to about 20carbon atoms. The number of fluoro groups on the benzene, and/or thefluoroalkyl group, is usually 1 to about 6, more usually, 1 to about 3.In general, the number of fluoro groups is sufficient to achieve apredetermined refractive index and boiling point. Examples ofsubstituted benzenes include, by way of example and not limitation,toluene, xylene, ethylbenzene, diethylbenzene, propylbenzene,dipropylbenzene, and so forth, including the ortho, meta and para formsthereof where appropriate. The substituted benzenes also includefluorinated derivatives of the above. Examples of fluorinatedderivatives, by way of illustration and not limitation, includefluorobenzene, difluorobenzene, trifuorobenzene, tetrafluorobenzene,pentafluorobenzene, hexafluorobenzene, fluorotoluene, difluorotoluene,trifluoromethyltoluene, tetrafluorotoluene, and pentafluorotoluene, andthe like.

The alkane of the combination is straight chain, branched chain, orcyclic alkane or a combination thereof. The alkane usually comprises 1to about 30 carbon atoms, more usually, about 1 to about 8 carbon atoms.The alkane may be unsubstituted or substituted with one or moresubstituents, usually 1 to about 3 substituents, which may beindividually selected. The substituents may be a hydroxy group, an oxogroup (double bond to oxygen), a keto group (C═O), alkoxy group, and thelike. The number of substituents is generally sufficient to achieve apredetermined refractive index and boiling point for the alkane.Particular examples of alkanes in accordance with this aspect of theinvention include, by way of illustration and not limitation,cyclohexane, cyclopentane, hexane, pentane, butane, propane, neopentane,methylbutane, methylpropane, methanol, ethanol, 2-propanol, 1-propanol,2-butanol, 2-methyl-2-propanol, 2-methyl-1-propanol, acetone, butanone,cyclohexanone, cyclopentanone, and the like.

Examples of liquid compositions in accordance with this aspect of theinvention, by way of illustration and not limitation, are set forth inTable 3.

TABLE 3 Composition Weight % Benzene/cyclohexane 80-85/15-20Benzene/cyclopentane 80-85/15-20 Benzene/hexane 85-90/10-15Toluene/cyclopentane 80-85/15-20 Toluene/fluorobenzene 45-50/50-55Thiophene/cyclohexane 60-65/35-40 Benzene/hexafluorobenzene 40-55/45-60Benzene/fluorobenzene 45-50/50-55

The liquid compositions of this aspect of the invention have relativelystable refractive indices and relatively low boiling points. Thecompositions are stable to thermal or other decomposition and do notpolymerize to any appreciable extent under the conditions of use. Thecompositions have a rate of change of refractive index with wavelengthin the 1500 nm region that is close to that of silica.

One aspect of the present invention involves a method for transmittingoptical signals. As mentioned above, signal exchanges withintelecommunications networks and data communications networks may beaccomplished optically. In this regard, data exchange may be carried outby the transmission of optical signals through, for example, opticalfibers. Optical switches have been designed for use intelecommunications and data communications networks. In the presentmethods optical signals are generated by, for example, diode lasers,erbium-doped amplifiers, and the like. The generated optical signals aretransmitted along a pathway of an optical device such as, for example,an optical fiber, an optical waveguide, an optical switch, and so forth.The pathway contains a refractive index-matching liquid wherein therefractive index-matching liquid is a liquid composition as discussedabove.

In the present invention the aforementioned liquid composition isinterfaced with a solid component of a device. As mentioned above, thepresent refractive index matching liquid compositions may be employed ina wide variety of devices wherein a refractive index matching liquid isemployed. These devices include, by way of illustration and notlimitation, optical fibers, optical waveguides, bubble in trenchdevices, thermal capillary switches, thermal optical waveguides, opticalgate switches, optical windows, and the like. Typically, the devicescomprise a solid component that in turn comprises a material such as,for example, silicon dioxide (silica), silicon dioxide with variousadditives often referred to as glasses, and the like or any of the abovein combination with one another or with various layers such as, forexample, oxide layers, polymer layers, and the like. A common featureamong the devices is a solid component that is in contact with orinterfaced with a liquid composition. It should be noted that theaforementioned ranges for refractive indices are primarily directed tosolid components that comprise silicon dioxide and/or silicon dioxidewith various additives. Where other solid components are employed suchas, for example, silicon, the aforementioned ranges may requireadjustment based on the refractive index matching requirements of thesolid component.

The following specific devices are discussed next by way of illustrationand not limitation.

One example of a device is that depicted in FIGS. 1 and 2. A switchingelement 10 is shown In FIG. 1, while a 4×4 matrix 32 of switchingelements is shown in FIG. 2. The optical switch of FIG. 1 is formed on asubstrate, which may be a silicon substrate or other suitable materialas discussed above. Optical switch 10 includes planar waveguides definedby a lower cladding layer 14, a core 16 and an upper cladding layer 18.The core is primarily silicon dioxide, but may comprise other materialsthat affect the index of refraction of the core. The cladding layers areusually formed of a material having a refractive index that issubstantially different from the refractive index of the core materialso that optical signals are guided along the core material.

The core material 16 is patterned to divide an input waveguide 20 and anoutput waveguide 26 of a first waveguide path and to define an inputwaveguide 24 and an output waveguide 22 of a second waveguide path. Atrench 28 is etched through the core material to the silicon substrate.The waveguides intersect the trench at an angle of incidence greaterthan the critical angle of total internal reflection (TIR) when thetrench is filled with vapor or gas. Thus, TIR diverts light from theinput waveguide 20 to the output waveguide 22 unless an index-matchingmaterial is located within the gap between the aligned segments 20 and26. Ideally, trench 28 is positioned with respect to the four waveguidessuch that one sidewall of the trench passes directly through theintersection of the axis of the waveguides. In the 4×4 matrix 32 of FIG.2, any one of four input waveguides 34, 36, 38 and 40 may be opticallycoupled to any one of four output waveguides 42, 44, 46 and 48. Theswitching arrangement is referred to as “non-blocking” since any freeinput fiber can be connected to any free output fiber regardless of whatconnections have already been made through the switching arrangement.Each of the sixteen optical switches has a trench that causes TIR in theabsence of an index-matching liquid, but collinear segments of aparticular waveguide path are optically coupled when the gaps betweenthe collinear segments are filled with an index-matching fluid. Trenchesin which the waveguide gaps are filled with fluid are represented byfine lines that extend at an angle through the intersections of opticalwaveguides in the array. On the other hand, trenches in which there isan absence of index-matching fluid at the gaps are represented by broadlines through a point of intersection.

The input waveguide 20 of FIGS. 1 and 2 is in optical communication withthe output waveguide 22 as a result of reflection at the empty trench28. Since all other cross points for allowing the input waveguide 34 tocommunicate with the output waveguide 44 are in a transmissive state, asignal that is generated at input waveguide 34 will be received atoutput waveguide 44. In like manner, input waveguide 36 is opticallycoupled to the first output waveguide 42, the third input waveguide 38is optically coupled to the fourth output waveguide 48, and the fourthinput waveguide 40 is coupled to the third output waveguide 46.

In accordance with the present invention the aforementioned liquidcompositions, i.e., Refractive Index-matching Liquids Type 1, Type 2 orType 3, may be employed as the refractive index-matching fluid of thedevices of FIGS. 1 and 2. The liquid compositions of the invention maybe employed in the gap located between the aligned segments 20 and 26 ofthe devices of FIGS. 1 and 2.

Another device to which the present invention has application is thatdescribed in U.S. Pat. No. 5,699,462 (Fouquet, et al.). A switchingelement defines a transmitting state and a reflecting state for a pairof intersecting waveguides that have a gap at their intersection. In thepreferred embodiment, the switching element exhibits total internalreflection at the gap sidewall from one waveguide to the other when notin the transmitting state. In the transmitting state, index-matchingliquid fills the gap, enabling light to continue in the originalwaveguide direction. The switching element may use inkjet techniques orbubble techniques to displace index-matching liquid. The index-matchingfluid may be projected from a gap between the waveguides by a jetmechanism; or a vapor or dissolved gas bubble may be formed to fill thegap between the waveguides to provide the reflecting state for theswitching element. Using either of these techniques, heaters areemployed to initiate the switching operation.

U.S. Pat. No. 6,195,478 B1 (Fouquet) discloses a planar light wavecircuit-based optical switch using micromirrors in trenches. A planarlight wave circuit is formed to include switching elements in whichoptical coupling among waveguides is determined by positions of adisplaceable member, such as micromirrors. Each switching elementincludes at least two light-transmitting waveguides extending along awaveguide substrate to a trench. The optical coupling between thewaveguides of a switching element is dependent upon the opticalcharacteristics exhibited at the trench. The displaceable device of aswitching element has a transmitting position and a reflecting position.The displaceable device may be manipulated using microelectromechanicalsystem techniques or techniques similar to those used in a dot matrixprinter engine. The trench at the crosspoint of waveguides may include aliquid having a refractive index that closely matches the refractiveindex of the core material of the waveguides. The present liquidcompositions of the types described above may be employed in thisdevice.

The liquids of the present invention may be used in devices thatrepresent changes to the device shown in FIGS. 1 and 2. For example,Jackel, et al. (U.S. Pat. No. 4,988,157), disclose an embodiment of adevice Wherein water or a refractive index-matching liquid resideswithin the gap between waveguides until an electrochemically generatedbubble is formed. A pair of electrodes is positioned to electrolyticallyconvert the liquid to gaseous bubbles. A bubble at the gap betweencollinear waveguides creates an index mismatch and causes light to bereflected at the sidewall of a trench. The bubble can be destroyed by asecond pulse of appropriate polarity, thereby removing the bubble andreturning the switch to the transmissive state.

Jackel (U.S. Pat. No. 4,988,157) discloses an optical switch usingbubbles. Parallel input waveguides and parallel output waveguides areformed on a substrate at perpendicular angles so as to intersect. A 45degree slot is formed across each intersection and is filled with afluid having a refractive index matching the waveguide material.Electrodes are positioned adjacent the slots and are selectivelyactivated to electrolytically convert the fluid to gaseous bubbles,thereby destroying the index matching across the slot and causing lightto be reflected by the slot rather than propagating across the slot. Inthe presence of a catalyst, a pulse of opposite polarity of sufficientsize and of he same polarity destroys the bubble. The liquidcompositions described above may be employed in such a device as therefractive index-matching liquid.

Another embodiment of a device is disclosed in Japanese patentapplication No. 6-229802 (Sato, et al.) (Kokai No. 8-94866). In thisembodiment heaters are employed to supply and remove index-matchingliquid to and from a gap that is intersected by two waveguides. Flow ofliquid is controlled by selectively activating heater elements. Thepresent refractive index-matching liquids may be employed as therefractive index-matching liquid used in the Sato embodiment.

A switch element having an expanding waveguide core is disclosed in U.S.Pat. No. 5,960,131 (Fouquet, et al.). The device comprises a switchingelement that selectively couples a first optical path to a secondoptical path through an index-matching fluid. The switching elementincludes a tapering region along each of the optical paths to achievehigh coupling efficiency at both ends of substrate waveguides that formportions of the two optical paths. The two substrate waveguides areseparated by a gap that is filled with the index-matching fluid in orderto optically couple the two waveguides. In accordance with the presentinvention the aforementioned liquid compositions, i.e., RefractiveIndex-matching Liquids Type 1, Type 2 or Type 3, may be employed as therefractive index-matching fluid of the Fouquet device.

Another device in which the present liquid compositions may be utilizedis disclosed by Sato, et al., in U.S. Pat. No. 6,072,924. Sato disclosesan optical switch that includes a substrate having therein opticalwaveguides made of silicon and a silicon layer deposited on its topsurface. A space is formed in the crossing portion of the opticalwaveguides, which is covered with a lid. Preferably, a groove is formedin a surface of the optical waveguide substrate or a bonding surface ofthe lid. After the lid has been bonded, the groove makes a passage thatcommunicates between the space and an outside. The passage is a pouringslit for pouring an index-matching liquid and is connected to the space,which acts as a driving slit in which the index-matching liquid moves.

EXAMPLES

The invention is demonstrated further by the following illustrativeexamples. Parts and percentages are by weight unless otherwiseindicated. Temperatures are in degrees Centigrade (° C.) unlessotherwise specified. The following preparations and examples illustratethe invention but are not intended to limit its scope.

Test Device

A test device similar to that described in U.S. Pat. No. 5,699,462(Fouquet, et al.) and U.S. Pat. No. 6,195,478 B1 (Fouquet) wasconstructed from a silicon heater chip, a glass waveguide chip, andoptical fibers. The device had four waveguides, which crossed fouradditional waveguides to form an optical crossbar switch. The device wasevacuated with a vacuum pump, and a degassed sample of the test liquidwas introduced. After filling, entry pipes to the device were closedwith a valve. Pressure inside the device was controlled by adjusting thetemperature of a chamber connected to the interior of the device, whichwas also partially filled with the liquid at the time of the initialintroduction of liquid into the device.

Diode lasers were used to introduce 1550 nm light into the fibersconnected to the device. The output from the device was monitored atboth the reflecting and transmitting (“out” and “drop”) ports byphotodiodes. A digitizing oscilloscope was used to record the electronicsignals from these diodes.

Example 1 Single Component: Cyclohexane

Cyclohexane was introduced into the device as described above. Heaterswere 60×11 μm. For a device temperature of 45° C. and pressure chamberat 55° C., 80 mw was required to be applied to the heater to achievereflection. Transition times were about 0.5 millisecond.

Example 2 Mixture: Benzene and Cyclohexane

A mixture of 85% benzene and 15% cyclohexane (by weight) was preparedand introduced in to the device as described above. Heaters were 60×11μm. For a device temperature of 45° C. and pressure chamber at 55° C.,78 mw was required to achieve reflection. Transition times were about0.5 millisecond.

Example 3 Mixture: Benzene and Fluorobenzene

A mixture of 45% benzene and 55% fluorobenzene (by weight) was preparedand introduced in to the device as described above. Heaters were 60×11μm; for a device temperature of 45° C. and pressure chamber at 55° C.,80 mw was required to achieve reflection. Transition times were about0.5 millisecond.

Example 4 Mixture: Thiophene and Cyclohexane

A mixture of 61.5% benzene and 38.5% cyclohexane (by weight) wasprepared and introduced in to the device as described above. Heaterswere 60×11 μm; for a device temperature of 45° C. and pressure chamberat 55° C., 80 mw was required to achieve reflection. Transition timeswere about 0.5 millisecond.

Example 5 Mixture: Benzene and Cyclopentane

A mixture of 83% benzene and 17% cyclopentane (by weight) was preparedand introduced in to the device as described above. Heaters were 60×11μm; for a device temperature of 45° C. and pressure chamber at 55° C.,80 mw was required to achieve reflection. Transition times were about0.5 millisecond

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be readily apparent to those of ordinary skill inthe art in light of the teachings of this invention that certain changesand modifications may be made thereto without departing from the spiritor scope of the appended claims. Furthermore, the foregoing description,for purposes of explanation, used specific nomenclature to provide athorough understanding of the invention. However, it will be apparent toone skilled in the art that the specific details are not required inorder to practice the invention. Thus, the foregoing descriptions ofspecific embodiments of the present invention are presented for purposesof illustration and description; they are not intended to be exhaustiveor to limit the invention to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to explainthe principles of the invention and its practical applications and tothereby enable others skilled in the art to utilize the invention.

1. A device comprising: (a) a solid component and (b) a liquidcomposition interfaced therewith, said liquid composition having arefractive-index that is substantially equal to that of said solidcomponent, said liquid composition comprising a fluorinated benzene or acombination of benzene and a fluorinated benzene.
 2. A device accordingto claim 1 wherein said liquid composition comprises a combination ofbenzene and a fluorinated benzene substituted with one to 6 fluorogroups attached to the ring.
 3. A device according to claim 1 whereinsaid fluorinated benzene is selected from the group consisting offluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene,pentafluorobenzene, hexafluorobenzene, fluorotoluene, difluorotoluene,trifluoromethyltoluene, tetrafluorotoluene, and pentafluorotoluene.
 4. Adevice according to claim 1 wherein said fluorinated benzene isfluorobenzene, 1,3-difluorobenzene, 1,3,5-trifluorobenzene,hexafluorobenzene, 2-fluorotoluene, 3-fluorotoluene or 4-fluorotoluene.5. A device comprising the device of claim 1 wherein said devicecomprises a groove in a substrate.
 6. An optical system comprising: (a)a device according to claim 1, said solid component comprising a cavityand an optical path comprising first and second segments separated bysaid cavity, and (b) control means for selectively causing said liquidcomposition to be disposed in said cavity between said first and secondsegments.
 7. An optical system according to claim 6 wherein saidfluorinated benzene is selected from the group consisting offluorobenzene, difluorobenzene, trifuorobenzene, tetrafluorobenzene,pentafluorobenzene, hexafluorobenzene, fluorotoluene, difluorotoluene,trifluoromethyltoluene, tetrafluorotoluene, and pentafluorotoluene. 8.An optical system according to claim 6 wherein said solid componentcomprises silica.
 9. An optical system according to claim 6 wherein saidsolid component is a substrate comprising a groove, said substrate beinga component of an optical switch.
 10. An optical switch comprising: (a)optical waveguides that are formed in a substrate and intersect eachother, (b) a cavity having a wall surface at a predetermined angle fromthe optical axis of the optical waveguide and positioned at theintersection of the optical waveguides, and (c) a refractiveindex-matching liquid disposed for selective introduction into saidcavity wherein the refractive index-matching liquid comprises afluorinated benzene or a combination of benzene and a fluorinatedbenzene.
 11. An optical switch according to claim 10 wherein saidfluorinated benzene is benzene substituted with one or more fluorogroups attached to the ring.
 12. An optical switch according to claim 10wherein said fluorinated benzene is selected from the group consistingof fluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene,pentafluorobenzene, hexafluorobenzene, fluorotoluene, difluorotoluene,trifluoromethyltoluene, tetrafluorotoluene, and pentafluorotoluene. 13.An optical switch according to claim 10 wherein said substrate comprisessilica and said cavity is a groove.
 14. An optical switch according toclaim 10 wherein said fluorinated benzene is fluorobenzene,1,3-difluorobenzene, 1,3,5-trifluorobenzene, hexafluorobenzene,2-fluorotoluene, 3-fluorotoluene or 4-fluorotoluene.